In a line-start permanent magnet (PM) motor having a rotor cage and a starting armature in addition to a PM armature, the starting armature is wound with at least one fewer pole-pair than the PM armature, and thereby does not couple with the rotor magnets. The starting armature accelerates the motor rotor by induction action on the rotor cage, and when the motor obtains synchronous speed, the power source is switched to the PM armature. Since the rotor magnets do not couple to the starting armature, large torque pulsations arising from non-synchronous magnet interaction are precluded during the starting period.
It was originally desired to have a centrifugal relay transfer the power source from the starting armature to the PM armature. At the moment that a centrifugal relay operates, the alignment of the motor back-emf phasors with respect to the power source phasor is random by nature. Dynamic simulations indicate that there are preferable alignments of the back-emfs at the moment of connection, and that non-preferable alignments will produce large torque pulsations, little different from the torque pulsations that the starting armature was intended to prevent. The large torque pulsations can be acoustically objectionable, and can cause mechanical failure of components. The electromagnetic conditions that cause the torque pulsations can also cause demagnetization of the magnets.
Use of a centrifugal relay which only detects the frequency of the PM armature back-emf for the PM motor starting function is not sufficient because synchronization depends on a correct phase relationship between line voltage and the PM armature back-emf. To a first approximation, a preferable synchronization occurs when the back-emf phasor of the PM armature regular phase (the phase without a capacitor) is aligned with the source phasor, and the worst synchronization occurs when the back-emf phasor of the regular phase is 180 degrees out of alignment with the source phasor.